The detection and measurement of substances, chemicals, or analytes in a bodily fluid sample is useful in a variety of applications, such as in fitness monitors or in the medical device industry. For example, an individual may choose to monitor a concentration of an analyte such as β-hydroxybutyrate or glycerol in his or her bloodstream in order to determine whether or not a chosen fitness regime is effective. Glycerol is a fitness related analyte associated with lipolysis and fat breakdown from stored body fat.
As another example, people with diabetes need to regularly monitor the concentration of glucose in their bloodstream in order to determine if they are in need of glucose or insulin or other diabetes medication. Diagnostic devices and kits have been developed over the years to allow a diabetic individual to autonomously determine the concentration of glucose in their bloodstream, in order to better anticipate the onset of hyperglycaemia or hypoglycaemia and take any necessary action.
When trying to ascertain a level of an analyte in, for example, a blood sample, an individual will typically perform a finger stick using a lancing device to extract a small drop of blood from a finger or alternative site. An electrochemical test device, which is often a test strip, is then inserted into a diagnostic meter, and the sample is applied to the test strip. Through capillary action, the sample flows through a capillary channel across a measurement chamber of the device and into contact with one or more electrodes or conductive elements coated with sensing chemistry for interacting with a particular analyte or other specific chemical (for example glucose) in the blood sample. The magnitude of the reaction is dependent on the concentration of the analyte in the blood sample. The diagnostic meter may detect the current generated by the reaction of the sensing chemistry with the analyte, and the result can be displayed to the individual.
Typically, such electrochemical test devices have a set of electrodes such as a counter/reference electrode and one or more working electrodes. Sensing chemistry is used which is typically tailored to the particular analyte or biometric of interest. An enzymatic electrode is a combination of an enzyme and an electrochemical transducer. The direct transfer of electrons between the enzyme and the electrode is generally not easy to achieve and so an electron transfer agent (or mediator) is sometimes used to carry electrons between the electrode and the enzyme to facilitate the electrocatalysis. For example, when measuring the concentration of glucose in a sample, a glucose oxidase or a glucose dehydrogenase enzyme can be used in conjunction with a mediator such as potassium ferricyanide. When detecting other analytes, different enzymes may be used, such as β-hydroxybutyrate dehydrogenase for measuring the ketone body β-hydroxybutyrate.
A number of different mediators, or electron transfer agents have been used in enzyme-based electrochemical sensors. Many of these mediators have one of more of the following limitations: they are poorly soluble and/or unstable in test solutions, they do not exchange electrons rapidly enough with the enzyme, electrode, or both. Some of these electron transfer agents also interact with interfering electroactive compounds, which lead to inaccurate measurements.
There is therefore a need for compositions comprising analyte-detecting enzymes and electron transfer agents which can be used as a part of an electrochemical test device and which enable accurate and sensitive analyte measurement.